<!doctype html>
<style>
p {display:table}

[x=a s]   { color:blue }
[rel=A s] { color:red }  /* values of some special attributes are matched case-insensitively by default https://html.spec.whatwg.org/multipage/semantics-other.html#case-sensitivity-of-selectors */
</style>

<p x=a>blue      <!-- this test fails in Chrome, works in Firefox -->
<p rel=a>black